The present invention relates to a displacement detector for an encoder well suited for detection of angular or positional displacement.
Various detectors have been developed and used in practice for detection of displacement through demodulation of output signals of an encoder. One typical example of such a conventional displacement detector is given in the form of an electronic phase-locked loop as later described in more detail. In summary, such a displacement detector includes a detection head made up of a pair of spaced magnetic sensors arranged facing a scale of an encoder. The scale is either circular or linear and includes a series of aligned magnetic domains of a similar period. The magnetic sensors and the scale are arranged for a prescribed relative movement.
During the relative movement, the magnetic sensors sequentially issue a series of phased output signal pairs in response to change in intensity of magnetic field generated by the magnetic domains of the scale of the encoder. After described signal processing through various circuit elements, the displacement detector issues a series of system outputs in the form of displacement data which indicate the instant position of the magnetic sensors with respect to the scale of the encoder. Each displacement data Dout is made up of output signals issued by a pair of counters in a parallel mode. The output signal of one counter forms higher bits of the displacement data Dout and indicates the instant position of the magnetic sensors with respect to the scale, i.e. the number of the magnetic domains passed by the magnetic sensors. Whereas the output signal of the other counter forms lower bits of the displacement data Dout and indicates the instant position of the magnetic sensors within a magnetic domain now being passed by the magnetic sensors.
It is now assumed with the above-described circuit of the displacement detector that the scale contains 1024 magnetic domains and the first-named counter has 8 bits. When the rotation speed of the scale is equal to 1 RPS (rotation per second), the frequency of the lowest bit of the first-named counter amounts to about 260 kHz.
A rotary encoder is required to have a function to perform exact detection of position even at a rotation speed of the scale in a range from 6 to 60 RPS. The above described circuit assures detection of position with high degree of dissolution when the rotation speed of the scale is relatively low. However, the circuit cannot follow well high speed rotation of the scale due to slow response of the phase locked loop and such poor function at high rotation speed degrades the accuracy of the count value .phi. of the first-named counter which indicates the instant position of the one magnetic sensor within a magnetic domain. Such inexact displacement data are issued from the circuit without any proper compensation.
In the case of the above-described circuit, the output signal N of the second-named counter and the count value .phi. of the first-named counter are issued in a parallel mode to form the displacement data Dout. As a consequence, the number of cables used for signal transmission must be equal or larger than the number of bits of the displacement data Dout. When the displacement data Dout is supplied outside the system in such a parallel mode, there is a great possibility of transmission error due to noise etc., thereby lowering reliability of the detection.
Further in the case of the above-described conventional circuit, the count values of the counters are not held when the power source is deactivated. As a result, no displacement data Dout can be issued right after activation of the power source. It is thinkable to use a proper external memory in order to provisionally store the displacement data Dout at deactivation of the power source. In this case, however, the displacement data Dout stored at the memory differs from the actual displacement should the scale move after deactivation of the power source.